Retroreflective sheeting for reflecting incoming light toward light source has been well known, and such sheeting with use of its retroreflective characteristics has been widely applied in the above-mentioned fields.
Generally, basic optical properties which are required of retroreflective sheeting include both high brightness in night time, i.e., the intensity of reflective brightness represented by reflective brightness against light which is being frontally incoming toward said sheeting, and wide angularity. With regard to wide angularity, there are demanded three properties, i.e., observation angularity, entrance angularity and rotational angularity. Good visibility of sheeting in day time is also required.
There have been reported some methods to improve the visibility of retroreflective sheeting in day time. A known one among them is to give fluorescent appearance by means of including fluorescent dye in some layer or other of retroreflective sheeting.
U.S. Pat. No. 3,830,682 of Rowland, for instance, discloses that fluorescent retroreflective sheeting of clear color tone is produced by compounding fluorescent dye such as Rhodamine B Extra, Rhodamine 6DGN, Fluorol 7GN and Amaplast Orange LFP in prism layer of triangular-pyramidal cube-corner retroreflective sheeting.
U.S. Pat. No. 5,387,458 of Pavalka discloses fluorescent retroreflective sheeting of clear color tone which is constituted of a screen layer which absorbs ultraviolet ray although substantially transparent to visible light, and of triangular-pyramidal cube-corner retroreflective sheeting elements layer to which thioindigoid type-, thioxanthene type-, benzoxazole coumarin type- or perylene imide type-dye has been added.
U.S. Pat. No. 5,605,761 of Burns discloses fluorescent retroreflective sheeting of clear color tone wherein thioxanthone type-, perylene imide type- or thioindigoid type-fluorescent dye and hindered amine type light stabilizer have been added to triangular-pyramidal cube-corner retroreflective elements layer.
U.S. Pat. No. 5,672,643 of Burns discloses fluorescent retroreflective sheeting of color tone in a specific range wherein retroreflective elements layer of cube-corner retroreflective sheeting contains a combination of perylene imide type-fluorescent dye of a specific structure with a specific fluorescent dye which is selected from Lumogen F Yellow 083, CI Solvent Yellow 160:1, CI Solvent Green 4, CI Pigment Yellow 101, CI Solvent Yellow 131, CI Solvent Yellow 98, Oraset Yellow 8GF, CI Solvent Green 5 and Golden Yellow D304.
None of the fluorescent retroreflective sheeting which has been proposed in the above-mentioned patents simultaneously satisfies both requirements of excellent fluorescent appearance and weatherability to maintain fluorescent color.
Inventors of the present invention have made assiduous study about fluorescent dye which is usable for retroreflective sheeting, and which has both properties of excellent fluorescent appearance and weatherability to maintain fluorescent color for a long period of time. As a result, they have found out that a certain kind of benzimidazole coumarin type- and benzopyran type-fluorescent dyes have both of the above-mentioned properties, and have thus completed this invention.